Configuring Loop Detection; Overview; Loop Detection Mechanism - HPE FlexFabric 5940 Series Configuration Manual

Configuring loop detection

Overview

Incorrect network connections or configurations can create Layer 2 loops, which results in repeated
transmission of broadcasts, multicasts, or unknown unicasts. The repeated transmissions can waste
network resources and can paralyze networks. The loop detection mechanism immediately
generates a log when a loop occurs so that you are promptly notified to adjust network connections
and configurations. You can configure loop detection to shut down the looped port. Logs are
maintained in the information center. For more information, see Network Management and
Monitoring Configuration Guide.

Loop detection mechanism

The device detects loops by sending detection frames and then checking whether these frames
return to any port on the device. If they do, the device considers that the port is on a looped link.
Loop detection usually works within a VLAN. If a detection frame is returned with a different VLAN
tag than it was sent out with, an inter-VLAN loop has occurred. To remove the loop, examine the
QinQ or VLAN mapping configuration for incorrect settings. For more information about QinQ and
VLAN mapping, see
Figure 47 Ethernet frame header for loop detection
The Ethernet frame header for loop detection contains the following fields:
•
DMAC—Destination MAC address of the frame, which is the multicast MAC address
010f-e200-0007. When a loop detection-enabled device receives a frame with this destination
MAC address, it performs the following operations:
Sends the frame to the CPU.
Floods the frame in the VLAN from which the frame was originally received.
•
SMAC—Source MAC address of the frame, which is the bridge MAC address of the sending
device.
•
TPID—Type of the VLAN tag, with the value of 0x8100.
•
TCI—Information of the VLAN tag, including the priority and VLAN ID.
•
Type—Protocol type, with the value of 0x8918.
Figure 48 Inner frame header for loop detection
"Configuring QinQ" and "Configuring VLAN
141
mapping."